1. Technical Field
The present invention relates to an antenna, and more specifically, to a feeding structure for providing an RF signal radiated from an antenna and the antenna using the feeding structure.
2. Background Art
An antenna is an apparatus for receiving RF signals in the air inside a terminal and transmitting signals inside the terminal to outside, and it is an indispensable element in communicating with outside in a wireless device.
FIG. 1 is a view showing the configuration of an antenna according to a conventional technique. Referring to FIG. 1, the antenna 10 according to a conventional technique includes a feeding unit 11 and radiators 12a and 12b. In the antenna 10 according to a conventional technique, the feeding unit 11 is directly connected to the radiators 12a and 12b, and a signal provided by the feeding unit 11 is transmitted to outside through the radiators 12a and 12b. At this point, the ground of a wireless communication device may be used as the radiators 12a and 12b, or the radiators 12a and 12b may be configured as a separate radiator. Alternatively, a separate radiator can be used as one of the radiators 12a, and the ground can be used as the other radiator 12b. 
In the antenna according to FIG. 1, since the feeding unit 11 directly provides electrical signals to the radiators 12a and 12b only in an electrical method without a separate feeding structure, performance of the antenna is lower than that of an antenna having a feeding structure.
FIG. 2 is a view showing an antenna having a feeding structure according to a conventional technique. Referring to FIG. 2, the antenna 20 according to a conventional technique includes a feeding unit 21, radiators 22a and 22b, and a conducting line 24 for forming a feeding loop 25.
The antenna 20 according to FIG. 2 forms the feeding loop 25 using the conducting line 24, and thus feeding can be performed by magnetic coupling other than electrical feeding. Therefore, performance of the antenna is improved compared with that of the antenna 10 in FIG. 1 that does not have a feeding loop 25. However, although an antenna has the feeding loop 25, performance is degraded in a high frequency domain. This will be described below in detail.
If RF current provided by the feeding unit 21 flows through the feeding loop 25, equivalent magnetic current is generated. The equivalent magnetic current Im is expressed as shown in mathematical expression 1.Iml=jωμSI(ω)  [Mathematical expression 1]
In mathematical expression 1, Iml denotes equivalent magnetic current having length l, ω denotes an angular frequency of RF current, μ denotes permeability, S denotes an area of a feeding loop, and I(ω) denotes RF current provided by the feeding unit.
The equivalent magnetic current Im generated in the feeding loop 25 can be considered as magnetic flux generated in the feeding loop 25, and the magnetic flux generated in the feeding loop 25 and the equivalent magnetic current Im have a relation as shown in mathematical expression 2.Im=jωψ  [Mathematical expression 2]
In mathematical expression 2, ψ denotes total magnetic flux generated in the feeding loop 25.
On the other hand, the total magnetic flux generated in the feeding loop 25 can be expressed as shown in mathematical expression 3.
                    ⁢          [              Mathematical        ⁢                                  ⁢        expression        ⁢                                  ⁢        3            ]            ψ    =                            ∫                                    B              →                        ·                                          ⅆ                s                            →                                      ≈                  B          ·          S                    =                        L          ·          I                =                              L            ⁢                          V                              R                +                                  j                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                  L                                                              ∝                      1            ω                              
According to mathematical expression 3, it is understood that as the frequency of the RF current provided by the feeding unit 21 increases, the amount of total magnetic flux generated in the feeding loop 25 is decreased. That is, decrease in the amount of total magnetic flux generated in the feeding loop 25 means decrease in the equivalent magnetic current Im. Accordingly, since the equivalent magnetic current Im decreases at a high frequency and thus RF signals cannot be efficiently fed to the radiators 22a and 22b, performance of the antenna shown in FIG. 3 is degraded at a high frequency, and thus the frequency band is narrowed.
Nevertheless, antennas of the conventional technique do not propose an efficient feeding structure for improving performance of an antenna, and it has been tried mainly to design an antenna having good characteristics by changing design of the radiator.
However, if the shape of a radiator is changed to be complex in order to improve characteristics of an antenna, manufacturing cost will be increased, and it is unable to correctly grasp which feature of an antenna is changed by which element of the radiator, and thus antenna design itself is getting further complicated and difficult.